Electrochemical analytical sensors are commonly used to determine the presence and concentration of a biological analyte. Such sensors are used, for example, to monitor blood glucose levels in diabetic patients.
The measurement of glucose in blood at home by people with diabetes is important in the management of their disease. The blood glucose is measured using meters and sensors, which are typically thrown away after each use. Various designs of sensors are known. For example, U.S. Pat. Nos. 5,120,420, 5,437,999, 6,143,164, 6,338,790 and 6,616,819 disclose disposable electrochemical sensors with a layered construction having electrodes present on the substrates.
Simplified, the electrochemical glucose strip uses two electrodes to measure the amount of glucose in blood. Additional electrodes may also be incorporated into strip designs.
However, the electrochemical glucose strip requires a minimum of two electrodes to perform a measurement. One electrode is the working electrode where an electrochemical reaction takes place that is proportional to the amount of glucose in the blood. The glucose reaction usually involves an enzyme that reacts with glucose, such as glucose dehydrogenase or glucose oxidase, and an electrochemically active compound capable of reacting with the enzyme and at the working electrode. Current cannot flow through the measurement circuit of the glucose meter unless an electrochemical reaction equal in size, but in the opposite direction to that occurring at working electrode, may be made to occur at a second electrode also in fluid contact with the blood. This second electrode has been referred to as a counter electrode or a counter/reference electrode. As explained below, the counter electrode must function properly so that the working electrode may accurately respond to the glucose in the blood.
The current generated at the working electrode by the reaction of glucose with enzymes and mediators is the desired reaction. Electrons flow out of the mediator and into the working electrode. The mediator is oxidized at the working electrode. At the counter electrode, an equal number of electrons must flow out of the counter electrode and react with a reactive compound somewhere near or on the counter electrode. The reactive compound is reduced at the counter electrode. The reaction at the counter electrode must be capable of generating electrons faster than the reaction at the working electrode so the reaction at the counter does not slow down the reaction at the working electrode. The counter electrode also typically acts as a reference electrode. As a reference electrode, the electrical potential of the counter remains relatively constant even though the reduction reaction is occurring at the counter electrode. A constant potential is typically applied between the working electrode and counter electrode. The applied potential sets the energy level of the electrons within the working and counter electrodes. The energy level of the electrons determines what reactions may occur at each electrode. The potential applied by the meter between the working and counter electrodes typically remains constant throughout the glucose measurement. However the meter, to which the sensor is connected, may only control the difference in potential between the two electrodes. The absolute potential is therefore a function of the reactions that occur at the working and counter electrodes. It is therefore generally desirable that the counter electrode remains at a fixed potential so that the working electrode will also remain at a fixed potential.
There are two main methods used to fix the potential at the counter electrode. In some designs, a highly concentrated mediator solution is provided over the working and counter electrodes. The concentrated mediator dissolves in the blood applied to the glucose strip. The concentrated mediator will partially stabilize the potential of the counter electrode based on the ratio of the oxidized and reduced concentrations of the mediator. The potential of the counter may be calculated using the well know Nernst equation, which will not be explained here. As the oxidation reaction progresses at the working electrode, mediator is reduced at the counter electrode to complete the current circuit. As a result, the concentrations of oxidized and reduced mediator are changing at the counter and as a result the potential of the counter is changing. This method of constructing a counter electrode can, at best, form a quasi-reference electrode. The amount of mediator that has to be used is also very high because it must exceed the concentration needed for the oxidation reaction at the working electrode. Glucose in blood may approach 40 mM so typical mediator concentrations for this type of electrode is often over 100 mM. Many of the best mediator compounds are not soluble at high concentrations such as 100 mM so they cannot be used for this type of glucose strip construction. Most of the mediator ends up dissolved in the solution far away from the counter/reference electrode and serves no useful purpose. In this type of counter electrode, the shifting potential may be overcome by increasing the potential applied by the meter to compensate for any shift that may occur at the working and counter electrodes. Increasing the applied potential however bring the risk of reacting with electrochemically active interferences in the blood and generally results in poorer accuracy of the glucose measurement.
The one advantage to this type of glucose strip construction is that the same metal may be used for both the working electrode and the counter electrode. The mediator, in solution, needs to react equally well at both the working and counter electrodes so making them out of the same material is a good choice. Various currently available glucose sensors are made by coating sheets of plastic with pure metals, such as gold or palladium, and forming in the desired shapes of the working and counter electrodes. Despite the efficiency of manufacturing using only one metal, this type of manufacturing is limited to using highly soluble mediators to set the potential of the counter electrode and serve as a source of electrons to support the current needed at the working electrode.
Another common type of available counter electrode uses the solid and insoluble material silver chloride. The silver chloride may easily be reduced to silver providing a ready source of electrons to complete the reaction. Because the silver chloride is in the form of a solid, it is dense and contains a large amount of silver chloride per area. The silver chloride may be placed in close proximity to the counter electrode so it may react quickly as well as it is insoluble so nothing is wasted by dissolving in the blood sample and moving away from the counter electrode. Silver chloride is usually used in the presence of silver metal. Silver chloride and silver metal provide a stable fixed potential so the counter electrode serves as a good reference electrode. The common method of manufacturing the counter electrode is to use screen printing. Screen printing inks are sold that contain both silver and silver chloride. The silver is a good conductor of electricity but silver chloride is not. The counter electrode must pass current so the screen printing inks require a significant amount of silver to remain conductive. The amount of silver chloride must be restricted or the electrical resistance of the counter will become too high and resist the flow of the electrical current. Using a screen printed Ag/AgCl for the counter electrode does not involve the mediator in generating current at counter electrode. Because the mediator is not needed at the counter electrode, low concentrations of mediator may be used. Low solubility mediators that provide superior performance to high solubility mediators may be used in this design. Additionally, it is only necessary to place the mediator over the working electrode and not both electrodes, which saves in cost of the mediator and the complexity of the manufacturing process. In many ways the screen printed Ag/AgCl counter electrode is superior to the counter electrode made from gold that uses high levels of soluble mediator to function.
Since analyte monitoring, e.g., glucose monitoring, continues to be of importance, there continues to be interest in sensors and the manufacture of sensors.